In the semiconductor art it is often desirable to form insulating layers or films on various supporting structures, such as gate insulators in field effect transistors, a passivation layer or protecting film covering various areas of electronic and optoelectronic devices, etc. Regardless of its use, it is imperative that the dielectric layer or film be a good insulator with low defect density to enable device operation and enhance device performance.
In prior art, insulating silicon nitride layers or films are routinely fabricated on Si substrate using chemical vapor deposition (CVD) techniques with silane (or dichlorosilane) and ammonia as source materials. The substrate is typically held at temperatures in excess of 700.degree. C. for silicon nitride deposition to allow efficient cracking of the silane gas. The reaction occurring in the process depends on the gas used and is either, EQU 3SiH.sub.4 +4NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +12H.sub.2, or(1) EQU 3SiCl.sub.2 H.sub.2 +4 NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +6HCl+6H.sub.2.( 2 )
Prior art also describes the deposition of silicon nitride at low temperature (&lt;400.degree. C.) using plasma systems in which silane reacts with a nitrogen discharge from a radio frequency (RF) or electron cyclotron resonance (ECR) source to form a silicon nitride. Silicon nitride films or layers deposited using a low temperature CVD process are hydrogen rich; they may contain more than 10% hydrogen dependent on deposition technique and conditions. This is a serious problem for a gate insulator. Hydrogen acts as a trapping center for holes (bulk trap density N.sub.t &gt;10.sup.12 cm.sup.-2) causing significant shift of device parameters such as threshold voltage and operating point. Further, the involvement of hydrogen in hot electron degradation and dielectric breakdown has been studied for decades. Degradation and subsequent breakdown are based on hot-electron-induced release of atomic hydrogen and subsequent hydrogen-induced defect generation leading to hydrogen induced build-up of fast interface states (D.sub.it) and slow trap states (anomalous positive charge), see for instance J. H. Stathis et al., Proc. International Conf. on Solid State Devices and Materials, pp. 791-793, Yokohama, Japan (1996).
Further, prior art, for instance U.S. Pat. No. 5,256,205, entitled "Microwave Plasma Assisted Gas Jet Deposition of Thin Film Materials", issued Oct. 26, 1993, and U.S. Pat. No. 5,356,672 entitled "Method for Microwave Plasma Assisted Supersonic Gas Jet Deposition of Thin Films, issued Oct. 18, 1994, reported the fabrication of virtually hydrogen free nitride films with a low bulk trap density N.sub.t .ltoreq.10.sup.11 cm.sup.-2 using a jet vapor deposition process in which supersonic jets of the source gases are used to deposit a silicon nitride film or layer at room temperature. The described jet vapor deposition process for silicon nitride operates at a high pressure of .congruent.1 Torr.
For compound semiconductors such as GaAs, an amorphous insulating layer or film with low trap density (N.sub.t .ltoreq.10.sup.11 cm.sup.-2) and high dielectric constant (&gt;3.9) is highly desirable for gate insulator applications. The insulating material such as silicon nitride can be directly deposited on a semiconductor wafer structure or alternatively, the silicon nitride film can be deposited on an amorphous oxide film which provides low interface state density D.sub.it on GaAs based semiconductors, see for example M. Passlack et al., Appl. Physics Lett., vol. 68, 1099 (1966). However, a high temperature deposition process is prohibitive on III-V (or even II-VI) compound semiconductor based structures (viz. the III-arsenides, III-phosphides, III-antimonides, II-tellurides among others) since compound semiconductors decompose at temperatures above 700.degree. C. Further, a high pressure process (pressure .gtoreq.10.sup.-4 Torr) cannot be integrated into an ultra-high vacuum (UHV) system without using an additional buffer chamber and thus, said high pressure process is incompatible with III-V semiconductor growth and surface preparation schemes required for gate insulator and surface passivation applications.
Accordingly, it would be highly desirable to provide new methods of manufacturing which overcome these problems.
It is a purpose of the present invention to provide a new and improved method of fabricating a silicon nitride layer or film.
It is another purpose of the present invention to provide a new and improved method of fabricating a silicon nitride layer or film which has a significantly reduced level of hydrogen.
It is still another purpose of the present invention to provide a new and improved method of fabricating a silicon nitride layer or film with a low trap density N.sub.t .ltoreq.10.sup.11 cm.sup.-2.
It is yet another purpose of the present invention to provide a new and improved method of fabricating a silicon nitride layer or film which is compatible with a low temperature process.
It is a further purpose of the present invention to provide a new and improved method of fabricating a silicon nitride layer or film using a molecular beam of high purity elemental Si and an atomic beam of high purity nitrogen.
It is still a further purpose of the present invention to provide a new and improved method of fabricating a silicon nitride layer or film on a compound semiconductor wafer structure.
It is yet a further purpose of the present invention to provide a new and improved method of fabricating a silicon nitride layer or film which is compatible with the requirements of III-V compound semiconductor manufacturing.
It is still a further purpose of the present invention to provide a new and improved method of fabricating a silicon nitride layer or film which is compatible with the UHV requirements of compound semiconductor growth and surface preparation schemes applied to gate insulator and surface passivation applications.
It is yet a further purpose of the present invention to provide a new and improved method of fabricating a silicon nitride layer or film wherein the substrate deposition temperature is below the decomposition temperature of the compound semiconductor wafer structure.
It is still a further purpose of the present invention to provide a new and improved method of fabricating a silicon nitride layer wherein the silicon nitride layer is fabricated on a stoichiometric upper surface of a compound semiconductor wafer structure.
It is yet a further purpose of the present invention to provide a new and improved method of fabricating an insulator-semiconductor structure with improved stability and reliability.
It is still a further purpose of the present invention to provide a new and improved method of fabricating a silicon nitride layer or film on semiconductor wafer structure which is relatively easy to fabricate and use.